Light amplifier device having an ion and low energy electron trapping means

ABSTRACT

A light amplification device such as an image intensifier or low level sensing device is disclosed which includes a photocathode spaced from an aluminized target electrode and a microchannel plate intermediate said cathode and target. A thin non-selfsupporting substantially optically transparent layer of material of a substance and thickness so as to be essentially transparent to high energy electrons, on the order of 100 to 1,000 electron volts, and light is situated atop the front end of the microchannel plate, covering the passages therein, in order to trap ions, which otherwise would travel to the photocathode, neutral gas ions, and to absorb scattered low energy electrons generated by secondary emission at the rim portion of the individual tubes in said microchannel plate, which would otherwise travel into the microchannel plate passages and to pass any light which passes through the photocathode and transmit any light which penetrates through the photocathode. The microchannel plate is spaced by a predetermined first distance from the photocathode, with its covered end facing the photocathode, and is spaced by a second distance, larger than the first distance, from the aluminized target electrode. A first voltage is applied between the photocathode and the microchannel plate and a second voltage, at least twice as great as the first voltage, is applied between the microchannel plate and the target electrode, and a third voltage is applied across the microchannel plate.

United States Patent 1 Einstein LIGHT AMPLIFIER DEVICE HAVING AN ION ANDLOW ENERGY ELECTRON TRAPPING MEANS [75] Inventor: Bernard CaesarEinstein, Redwood Estates, Calif.

[73] Assignee: Litton Systems, Inc., San Carlos,

Calif.

[22] Filed: Mar. 23, 1972 [21] Appl. No.: 237,343

Related US. Application Data [63] Continuation-in-part of Ser. No.124,107, March 15,

l97l,abandoned.

[30] Foreign Application Priority Data Feb. 29, 1972 Germany P 22 09533.6

' Feb. 29, 1972 Germany P 72 07 607.4 Mar. 1, 1972 Great Britain9,642/72 Mar. 10, 1972 Netherlands 7203218 [52] US. Cl 250/213 VT,313/103 [51] Int. Cl. H01] 31/50, HOlj 39/12, HOlj 43/00 [58] Field ofSearch 250/213 VT, 207;

[56] References Cited UNITED STATES PATENTS 3,603,832 9/1971 Manley,250/213 VT 3,660,668 5/1972 Wolski 250/213 VT Primary Examiner.lames W.Lawrence Assistant Examiner-T. N. Grigsby Att0mey-Ronald M. Goldman eta].

[ June 26', 1973 [57] ABSTRACT A light amplification device such as animage intensifier or low level sensing device is disclosed whichincludes a photocathode spaced from an aluminized target electrode and amicrochannel plate intermediate said cathode and target. A thinnon-self-supporting substantially optically transparent layer ofmaterial 'of a substance and thickness so as to be essentiallytransparent to high energy electrons, on the order of 100 to 1,000electron volts, and light is situated atop the front end of themicrochannel plate, covering the passages therein, in order to trapions, which otherwise would travel to the photocathode, neutral gasions, and to absorb scattered low energy electrons generated bysecondary emission at the rim portion of the individual tubes in saidmicrochannel plate, which would otherwise travel into the microchannelplate passages and to pass any light which passes through thephotocathode and transmit any light which penetrates through thephotocathode. The microchannel plate is spaced by a predetermined firstdistance from the photocathode, with its covered end facing thephotocathode, and is spaced by a second distance, larger than the firstdistance, from the aluminized target electrode. A first voltage isapplied between the photocathode and the microchannel plate and a secondvoltage, atleast twice as great as the first voltage, is applied betweenthe microchannel plate and the target electrode, and a third voltage isapplied across the microchannel plate.

27. Claims, 3 Drawing Figures l TIP i 2/ i- 3 E 4 5 l I l I 13 I I LPATENIEDJUNZS ms 3. 742.224

FORM

BAKE LACQUER IN AIR LAYER PLACE DEPOSIT LACQUER V ALQN ON PLATE LACQUER24 .LJ 1 SUCTION I F 3 TORETAIN I LAYER I L.

LIGHT AMPLIFIER DEVICE HAVING AN ION AND LOW ENERGY ELECTRON TRAPPINGMEANS This application is a continuation-in-part of my earlier filedapplication for patent, Ser. No. 124,107, filed Mar. 15, 1971 nowabandoned.

This invention relates to light amplification devices and, moreparticularly, to image intensifiers and low light level sensing devices.

BACKGROUND OF THE INVENTION Light amplification devices and,particularly, image intensifiers and low light level sensing devicesfind application for surveillance in all situations where limitedlighting is available and, particularly, in those applications where theonly available light is moonlight or starlight, lighting conditionswhich to the ordinary person with the unassisted eye are usuallyconsidered almost complete darkness. Under these conditions a primeapplication of such apparatus is to detect intruders in civilian areasurvelliance or to detect and locate enemy presonnel in battlefieldsurveillance. The apparatus can be designed to permit the information tobe acted upon locally or it can be designed to electronically relay thedetected images by means of conventional and wellknown televisionapparatus to a remote point.

In one conventional apparatus for those applications light amplificationdevices are incorporated within and made an integral part ofconventional television image transmitting camera tubes, such as thevidicon or image orthocon. In such a camera tube the light amplificationdevice serves as the front end which first detects and amplifies animage and places the amplified image upon an output traget electrode.The target electrode, in turn, is serially scanned in the camers portionand the image is converted into a series of serial electrical signalswhich may be transmitted to a remote point by cable or radio anddetected and displayed upon the cathode ray tube of a televisionreceiver.

Another conventional apparatus for the use heretofore mentionedincorporates the light amplification device in combination with opticalbinoculars or goggles for direct viewing so that guard or fieldpersonnel can observe areas under their personnel surveillance underconditions of almost complete darkness and act upon that information.

In still other equipments light amplifying devices can be incorporatedwithin cathode ray tubes to enhance the intensity of a reproduced imageto be displayed and reference may be made to prior art sources forspecific examples.

Basically, the operative elements of one common type of lightamplification device are found within an evacuated (in-vacuum) housingor tube with an optically transparent front window. The images receivedat the window are directed to a photocathode, a material which emitselectrons in proportion to the intensity of light incident thereon. Thelight image or pattern is projected upon a predetermined area of thephotocathode surface and a corresponding pattern of charges or electroncurrents are generated in the photocathode. The electrons of this mosaicof electron intensities are directed under the influence of a suppliedelectrostatic field to an electron multiplying element. The electronmultiplying element increases the number of traveling electrons and atits output provides a corresponding mosaic or image of electroncurrents. Under the influence of a second provided electrostatic fieldthese electrons are accelerated either directly or through an electronlens focusing system toward a target, suitably phosphor, which, becauseof the electron multiplication, provides a displayed or otherwise usableimage of a greater light intensity than the image received at tubewindow.

In one type of light amplifier device the electron multiplying elementis a transmission-emission dynode. The transmission-emission dynode isof a high secondary emission material. Thus those electrons travelingfrom the photocathode and incident upon the dynode cause the re-emissionof a larger number of secondary electrons.

A second conventional electron multiplying element uses as analternative to the dynode a microchannel plate. The microchannel platecomprises a bundle of very small cylindrical tubes packed togetherparallel and forming essentially a thick layer of material with a verylarge number of passages or openings through the layer. The innerpassage walls are coated with a high secondary emission material and anelectric static field is placed thereacross. The end surfaces of theplate are coated with a layer of electrically conductive material whichserve as electrodes. This coating does not cover the openings. Electronstraveling from a particular location on the photocathode are directed bythe electrostatic field to and enter a correspondingly located passagein the microchannel plate, and is incident upon the passage walls. Sincethe walls of each passage are coated or formed with a material having acharacteristic high. coefficient of secondary emission the incidentelectron knocks out, re-emits, from the wall surface at least two moreelectrons and they, in turn, travel in a general direction toward theend of the tube. By design, the length and diameter of the passage issuch that these electrons, in turn, again strike the passage walls atsubsequent locations and increase furtherthe number of electronstraveling toward the end of the passage. The increased number ofelectrons, hence amplified" electron intensity, exits from theindividual passage in the microchannel plate. Under the influence ofanother supplied electrostatic field the exiting electrons areaccelerated toward a corresponding location on the target electrode,typically a phosphor screen. By similar action at all other locations onthe photocathode and microchannel plate, a visual image or mosaicrepresentative of the original image received by the amplifier tube andfirst focused on the photocathode is displayed upon the targetelectrode.

By way of example of light amplifier devices and the possiblearrangements of the basic photocathodeelectron multiplier-targetelectrode and the additions and variations thereto and thereof thefollowing U.S. patents can beconsidered: U.S. Pat. Nos. 3,497,759;3,480,782; 3,478,213; 3,346,752; 3,345,534; 3,440,470; 3,387,137;3,513,345; r 3,528,101; and 2,903,596.

Because of its potentially greater amplification and performance, lightamplifier devices using a microchannel plate as the electron multiplyingelement are the present day choice in second. generation light amplifiertube structures. The device, however, has heretofore had severalsignificant limitations.

One limitation in the microchannel plate type light amplifier iscontrast resolution. As previously described, the microchannel plateelement consists of a bundle of small tubeforming passages. Each ofthese tubes has a rim or edge surrounding the passage opening. Even withthe multitude of passages through the plate the area taken up by thetube edges is on the order of fifty percent. Hence, electrons travelingtoward the microchannel plate are incident sometimes upon the tubeedges. Such electrons are bounced back or collide with other electronsat the plate surface and knock out from the surface one or more suchelectrons. These electrons first move in a direction against the nowopposed electrostatic field where they are decelerated and then areturned around under the influence of such electric field and thenaccelerated into the tube passages in the microchannel plate. In passinginto the microchannel plate these electrodes act like any other electronin normal operation as previously described. Unfortunately, there is anuncertainty as to into which one or more of the tube passages all or anyone of such electrons will proceed. in being knocked out from an edgesurface of one tube at a random velocity and direction the electron whenturned around under the influence of the electric field may go throughthat particular tube or any one of the closely packed adjacent tubes.

The reproduction quality of photographic image reproduction requiresthat a point of light received from one location be reproduced or imagedat a second location as a point. If, for some reason due to defects inthe optical system or otherwise, the light emitted from a point isscattered and reproduced at several closely adjacent points essentiallya blur is obtained. The limit of resolution can be determined in onemanner by locating another point of light closely adjacent the firstlight point and moving the two together. The

reproduced light images should be distinguishable and it is apparentthat if the two light sources show up at the image location as onesingle blur that the distance between light sources at which this occursis a limit to the resolution capability of the imaging system.

The electrons emitted from particular spacial locations on thephotocathode surface in the light amplifiers may be considered analogousto the previously described light points. Thus, if the electronsgenerated at the microchannel plate surface by electron collisions canflow into one or more adjacent passages somewhat randomly the outputimage of the intensifier essentially appears blurred. This means thatthe point source of electrons is not 100 percent accurately reproducedat the image location and that, with respect to the transfer functiondefined by the MC? resolution, degradation in performance results.

This characteristic is specified by those skilled in the art in terms ofthe number of raster or TV lines per unit of raster height which can beplaced upon the display screen and be discernible. As the number oflines per unit height are increased the more densely packed togetherthese lines become until the limit of resolution is reached: the pointat which time these adjacent lines merge together or blur and it isimpossible to determine the edge of any one line and the beginning ofany space between lines. As a result of the problem previously discussedpresent light amplifier devices have typically maximum resolutioncapabilities on the order of 400 TV lines per unit of raster height withthe raster height, in turn, being specified as four-tenths of 1 inch.

A further limitation is inherent. As is apparent to those familiar withtelevision type pickup and display apparatus, there are instances,however infrequent, where an electron current instead of colliding withthe wall and causing the emission of other electrons will, instead,cause the desorption of an atom as a positively charged ion, an atomicparticle that is of a mass several hundred times larger than anelectron. In the light amplifier device the original electrons aredirected to the microchannel plate under the influence of a largeelectric field the latter of which by the convention adopted points in adirection from the photocathode to the microchannel plate. This sameelectric field, however, is oriented such as to accelerate any suchlarge mass positive ions for travel in a direction toward thephotocathode. Thus the positive ions, when generated, travel to andcollide with the photocathode with deleterious effect. The structure andmethod of fabrication of microchannel plates is such that those positiveions derived from the microchannel plate structure may be, for example,water ions, H O or Cesium ions, Cs". ln colliding with the photocathodeconsisting of a different chemical substance, such as 8-20, the ion cancombine with the photocathode material to form compounds that do notpossess photocathode properties. And in colliding the kinetic energyreleased by the ion erodes the photocathode mechanically. Both due tothe release of large kinetic energy at the photocathode and the chemicalchanges caused to the photocathode, the photocathode deterioratesresulting in a serious loss of sensitivity in the amplifier tube,evidenced by a faded picture with diminishing birghtness and diminishingcontrast. As a result, the normal operating life'of the presentlyavailable microchannel type intensifier tubes in which images ofacceptable quality are provided may be on the order of 50 to hours.

The ion bombardment problem thus described is not significant in many ofthose light amplifying devices which use the transmission-emissiondynode electron multiplier structure. This is because the dynode acts asa trap for any ions generated due to electron incidence upon otherelements in the tube and the proximity of the dynode to the photocathodeprecludes large ion velocities. However, because of the other advantagesof the microchannel plate, primarily substantially higher gain, thelight amplifying devices using the microchannel plate are superior andare preferred.

lon bombardment problems are not unique to light amplifiers and arerecognized in the prior art with various means heretofore devised orproposed to minimize or eliminate the problem. One common example isprovided in television cathode ray tubes in which a magnetic fielddiverts the traveling positive ion to the side of the tube envelope.Absent this diversion the positive ion would proceed instead to thephosphor faceplate and gradually erode a spot in the middle of thescreen. Another example of a type of ion trap appears in a direct viewlight amplifier as disclosed in US. Pat. No. 3,350,594, issued Oct. 31,[967, to Davis, illustrating a light amplifier device of the firstgeneration type which does not include a microchannel plate orequivalent electron multiplying element. The approach therein suggestedis to coat the backside of the phosphor display screen with a porouscoating of aluminum atop the normal aluminum layer conventionallyapplied to the back of the screen for other purposes, the object beingto capture any positive ions produced at the phosphor screen within theporous layer so that they cannot travel back toward the photocathode.This structure appears to be necessitated because the light amplifyingdevice there shown simply does not incorporate any electron multiplieror multiplying elements such as a dynode or microchannel plate, whichelements would necessarily provide a large obstacle to the travel of thepositive ions from the phosphor screen back to the photocathode which,if included, minimizes this problem.

It also appears known to merely place a thick metal layer on and at thefront end of a microchannel plate merely to stop or trap positive ions.In the specification of U.S. Pat. No. 3,603,832 a low light levelamplifier tube structure is disclosed in which it is proposed to removethe conventional thick light opaque aluminum backing, the light and iontrap, located on the back of the phosphor target electrode or displayscreen, as variously termed, a combination referred to as an aluminizedscreen located spaced in back of the rear end of the microchannel plate,and to place that thick metal layer instead as an alternative at thefront end of the microchannel plate atop an insulator layer. in sodoing, patent US. Pat. No. 3,603,832 notes that the metal backing asapplied to the phosphor display screen in prior devices requires a veryhigh voltage, suitably on the order of 5,000 volts, to give thoseelectrons exiting the rear end of the microchannel plate sufficientkinetic energy and momentum to pass through the thick metal layerbacking to the phosphors of the display screen and the application ofsuch a large voltage requires a large physical spacing between thedisplay screen and the microchannel plate to avoid destructive voltagearc-overs. Further according to that patent, the large physical spacingbetween the aluminized display screen and the microchannel plate reducedthe light output intensity from the phosphor display screen and causedother undesired optical effects. As a compromise the patent henceproposed to remove the thick aluminum layer from the phosphor displayscreen, which permits a lower accelerating voltage between themicrochannel plate and the phosphor display screen, from 5,000 voltsdown to 1,500 volts, by example, and position the display screen moreclosely to the exit of the microchannel plate. Hence the light intensityoutput from the electron bombarded phosphors in the display screen isthe same as or better than in preceding devices. Because some ion trapis necessary the cited patent suggests locating the thick metal layer atthe front end of the microchannel plate and this required additionalmodifications to the prior devices. Namely as taught in the cited patentthe physical distance between the front end of the microchannel plateand the photocathode is increased and the accelerating voltage betweenthe photocathode and the front end of the microchannel plate is alsoincreased, from a low voltage of perhaps 1,000 volts to a high voltageof 5,000 volts, in order to sufficiently accelerate electrons to passthrough the repositioned thick metal layer, just as in the case of theprior art device where the metal layer was located on the back of thephosphor display screen. In so doing, U.S. Pat. No. 3,603,832 effects acompromise or selection in location of a tube element, the thick metallayer, rather than proposing an entirely new device. There is nodisclosure that a metal layer can be placed on both the front end of themicrochannel plate and the back of the phosphor display screen or thatthe distance between photocathode and microchannel plate should remainsmall and the accelerating voltage therebetween should remain low, sothat the low energy secondary electrons created on the input end surfaceof the microchannel plate can be reabsorbed by the trap layer andincrease resolution, and the number of elastically scattered secondaryelectrons, which do have the higher energy and can penetrate intorandomly located adjacent microchannel plate holes, is kept to aminimum; or that low energy electrons can be absorbed by a metal layerin combination therewith to provide substantially increased contrastresolution and noise reduction and whereas by increasing the voltagebetween the photocathode and microchannel plate more higher energysecondary electrons are being generated which would not be absorbed bythe metal layer and which would therefore decrease contrast resolution.

A further limitation is inherent in the nature of the photocathodeitself. While characterized as an opaque element, consideredquantitatively it is approximately percent opaque and can actuallytransmit as much as 10 percent of the incident light. Should light passthrough the photocathode and be incident upon the metal layer located atthe front end of the microchannel plate, that light canbe reflected fromthe metal layer back to the photocathode and circulates between themetal plate and the photocathode to generate improperly positionedelectrons by further photocathode emission. And thus a point source oflight becomes displayed as an enlarged point on the phosphor screen, aphenomenon characterized as blooming.

OBJECTS OF THE INVENTION Accordingly, it is an object of the inventionto provide an improved light amplifier device.

It is an additionalobject of the invention to provide a light amplifyingdevice having improved contrast resolution capabilities.

It is a still additional object of the invention to provide a new lightamplifier device having improved life and resolution capabilities andavoids blooming.

It is a still further object of the invention to minimize or eliminateentirely in a light amplifier tube positive ion bombardment of thephotocathode and eliminate low energy electrons without reducing theamplifier gain.

And it is still another object of my invention to increase theoperational life of and the quality of performance during that life of alight amplifier tube.

SUMMARY OF THE INVENTION In accordance with the foregoing objects of theinvention, the invention comprises in a light amplification device aphotocathode spaced from a target electrode, suitably a phosphor screenbacked by a thick metal layer, and a microchannel plate type of electronmultiplier between the cathode and screen, with the plate spaced morenear to the cathodethan to the screen. A very thin optically transparentlayer of a matherebetween. A substantially larger voltage in the rangeof 3,000 to 8,000 volts is applied between the rear end of themicrochannel plate and the metal backed phosphor screen to establishtherebetween an electric field gradient, suitably in the range of 3 X 10to 6 X l volts/cm. In accordance with the invention, the covering metallayer absorbs or dissipates scattered low energy electrons generated bysecondary emission at the front surface of the microchannel plate andthereby prevents such electrons from entering the plate passages.Additionally, the covering layer acts as a barrier to positive ionstraveling from the microchannel plate in the direction of thephotocathode and to any existent neutral gas atoms. And any light whichpasses through the substantially opaque photocathode is permitted topass through the metal layer rather than allowed to create multiplereflections in the space between the photocathode and metal layer.

The foregoing and other objects and advantages of my invention togetherwith modifications, substitutions and equivalents thereof and othervariations and additional advantages thereto become more readilyapparent from consideration of the following detailed descriptiontogether with the figures of the drawing in which:

DESCRIPTION OF DRAWINGS FIG. 1 illustrates symbolically a lightamplifier device which embodies the principles of my invention;

FIG. 2 illustrates pictorially a small section, A, of the microchannelplate and metal layer used in connection with the illustrated embodimentof FIG. 1; and

FIG. 3 illustrates schematically the steps of manufacturing amicrochannel plate and layer combination in accordance with a novelmethod.

DETAILED DESCRIPTION OF INVENTION Inasmuch as all elements and detailsof light amplifiers, other than the improved element, are conventionaland known to those skilled in the art, FIG. 1 illustrates the basicelements of a direct view type light amplifier device, whichincorporates the invention, in symbolic form. The light amplifierincludes an envelope or housing 1 represented by the dashed lines,suitably glass or ceramic materials, and the inside of the housing is invacuum. The housing includes an optically transparent faceplate 2,represented by dashed lines, with which to permit entry of an opticalimage and, in the direct view tube, an optically transparent rear window4 through which to view the displayed amplified image.

The conventional electron or light optical system 3 is indicated by thedashed lines. This element as is well known can simply be a transparentspace with which to allow the light image to pass, or a complicated,though conventional, structure for converting the received Eight imageinto a source of electrons representative of that image, i.e., anelectron image and then into an intermediate display. Additionally, aconventional optical lens focusing system, not illustrated, may belocated in a conventional manner in front of faceplate 2.

A photocathode 5 is situated at the front end of the tube to receive thelight image upon its surface. Photocathode 5 is a circular disk having apredetermined area constructed and supported within the tube envelope ina well-known manner. Suitable photocathode materials are cesium andantimony and the preferred material is sodium potassium-cesium antimonycombination, commercially sold under the designation 8-20.

A target or display electrode 13 is located at the rear of theenvelope 1. By target electrode I refer generically to the last electronreceiving electrode in the light amplifier tube, whether it be a directdisplay type or storage type of tube. As is conventional in a displaytype of light amplifier the target electrode 13, usually referred to asthe screen" in a direct view tube, is usually of a circular disk-shapedgeometry and consists of a coating of an electroluminescent material,such as P-20 phosphor. Suitably the phosphor target is formed as acoating on the tube window 4. In turn the phosphor is conventionallybacked or coated with a thick electron permeable layer of aluminum l6,suitably 500 to 1000 A in thickness, to enhance its electricalconductivity and function as an electrode and to function as an ion trapand a light trap.

Spaced from and located in between photocathode 5 and traget 13, andsupported by conventional means, not illustrated, is a microchannelplate 7. The microchannel plate is a conventional type of electronmultiplier and is cylindrical in geometry. Microchannel plate 7 consiststypically of a plurality of glass tubes of small diameter closely packedtogether and fused into a unit. In the conventional case in excess of100,000 individual tubes are incorporated within and make up this plate.Each of the tubes contains a passage 9therethrough opening on boththefront and back faces or sides of the plate.Typically, the diameter ofthis passage is on the order of 2 mils and the openings approximate 50percent of the total face area. By conventional techniques the outeredges or rims of these tubes on both front and back sides are coatedwith a conductive metal, lead or lead oxide, not illustrated, to formelectrically conductive end surfaces for the microchannel plate and soas to place all the tube ends on the front and on the back sides,respectively, electrically in common. The inside walls 11 of the glasstubes in microchannel plate 7 are coated with a highly resistiveelectrically conductive material and a high secondary emission coating.Usually this comprises a lead and lead oxide coating conventionallyproduced by hydrogen reduction of lead oxide glass. While the internalcoating is considered electrically conductive it is highly resistive andon the order of 100 megaohms, and thus is not an electrical shortcircuit.

The embodiment of FIG. 1 may be modified to include an electron focusingarrangement in the space between the microchannel plate and target 13.Such elements are conventional and can be included by choice withoutdeparting from the invention.

Suitable conductor means symbolically illustrated in FIG. 1, and labeled8, l0, l2, and 14, provide electrically conductive paths from thephotocathode 5, front end of microchannel plate 7, back end ofmicrochannel plate 7, and target 13, respectively, to correspondingterminals on the exterior of tube envelope 1.

A thin film or layer 15 of metal, suitably aluminum, is attached orcoupled to the front end of microchannel plate 7 and covers the entirefront surface thereof and hence, covers the open ends of passages 9 inthe microchannel plate. The film layer is of a thickness, d3, preferablyin the range of 50 to 400 A, or A by way of specific example, and is ofa material which has a low atomic mass or low specific gravity of 1.0 to4.0. Suitably the layer 15 comprises aluminum which in this dimension issubstantially transparent to electrons having energies in excess ofseveral hundred electron volts while opaque, absorptive, or dissipative,however effected, of electrons having energies of less than 20 electronvolts and is transparent substantially to light. In addition thealuminum layer forms a positive barrier to relatively large mass largevolume positive ions and neutral gas atoms. One obvious addition to benoted at this point is to provide in addition a very thin aluminum oxideskin on the aluminum layer on the underside abutting the front face ofthe plate. The front of the layered microchannel plate is spaced by adistance, d1, as close as is practical to the rear of photocathode;within the range of 0.005-inches to 0.020-inches, and 0.012- inches byway of specific example. In one example the length of the microchannelplate with the layer is approximately 0.025-inches in length front toback. And the distance between the rear end of the microchannel plateand the metal backed phospher display screen 13, d2, is within the rangeof 0.030-inches to 0.050-inches preferably and is 0.03 8-inches in thisspecific example. For operation of the light amplifier suitable sourcesof electrical energy or bias supplies are provided and symbolicallyillustrated as batteries in FIG. 1. A battery 17 has its positivepolarity output connected to lead and its negative polarity outputconnected to lead 8 to place the battery voltage between thephotocathode and the microchannel plate. Typically the voltage of thissupply is on the order of 400 to 1,000 volts, particularly 600 volts, inorder to establish an electrostatic field of a predetermined intensitygradient between the front face of the microchannel plate andphotocathode 5, which field is represented symbolically in FIG. 1 by thearrow labeled E1. Such a gradient E1 is equal to Vl/dl. The gradient E1is preferably on the order of 32 X 10 volts per centimeter within therange of X 10 volts/cm to 40 X 10 volts/cm. A second bias source voltageis represented by battery 19. Battery 19 has its positive polarityterminal connected to lead 12 and its negative polarity terminalconnected to lead 10 of the light amplifier. Battery 19 may be of avoltage on the order of 300 to 1,000 volts, typically 800 volts. Thisplaces the battery voltage between the front and rear ends of themicrochannel plate and establishes an electric field of predeterminedintensity between the front and back ends of microchannel plate 7 in adirection toward the back of plate 7. This electric field is representedby the arrow and labeled E2 in FIG. 1 and is typically 12.6 X 10 voltsper centimeter. A third bias source voltage is represented by battery 21in FIG. 1. Battery 21 has its positive polarity terminal connected tolead 14 and its negative polarity terminal connected to lead 12 of thelight amplifier. This places the voltage of battery 21 between the rearof the microchannel plate and the aluminized phosphor screen 13. Battery21 provides voltages on the order of 3,000 to 8,000 volts, typically5,000 volts, which establishes a predetermined electric field betweenand in a direction from the back end of microchannel plate 7 toaluminized phosphor display screen 13. This field is represented in FIG.1 by the arrow and symbol E3 and is preferably within the range of 3 X10 to 7 X 10 volts/cm. In one specific example the gradient E3 is 5 X 10volts/cm.

An exploded view of a segment of the front end of microchannel plate cutout by the dashed lines in FIG. I, which are labeled is presented inFIG. 2 to assist in the explanation of the operation and effects ofinvention. Thus FIG. 2 illustrates the microchannel plate 7, several ofthe individual tubes 9 which form the microchannel plate, the walls 11of tubes 9, and the layer 15 situated atop microchannel plate 7.

As previously described, aluminum layer 15 is of a thickness of A.Layers of this minute thickness are not self-supporting and wouldcrumble and fall apart if an attempt were made to form such a layer,lift it, and place it upon the microchannel plate. Accordingly specialtechniques are necessary to satisfactorily couple to aluminum layer toone surface of plate 15.

One alternative is to form the layer in place on the microchannel plateby a conventional phosphor filming technique. Such a technique requiresthe immersion in water during processing of the plate but does notrequire the film 15 to be self-supporting.

A second approach is to take a thick self-supporting film of aluminumoxide produced by conventional means such as anodization and thenevaporate aluminum onto the front side of the aluminum oxide film. Thislayer can then be placed atop the microchannel plate. For adhesion thefilm and plate may be fused together by simply passing electricalcurrent between the film and the plate. In this the aluminum oxideremains and serves to give increasedtransmission-secondary emissionmultiplication.

I prefer, however, to fabricate the aluminum film by the novel methodillustrated in connection with FlG. -3. In this method a thinself-supporting film of lacquer (nitrocellulose) is first formed byconventional techniques. This step is represented in FIG. 3 as block 22.The lacquer film is then takenand placed atop the microchannel plate asrepresented by block 23. Preferably prior to placing the lacquer film inplace I connect a vacuum pump to the back side of the microchannel platein order to produce a suction at the front surface as is represented bythe dashed lines of block 24. When the lacquer film is placed atop themicrochannel plate the vacuum assists holding this film in place.Because the lacquer material is very thin and electrostatically chargedthe lacquer film adheres to the microchannel plate immediately and thevacuum pump is removed. The microchannel plate with lacquer layer isthen placed in a conventional bell jar for aluminum deposition.

By conventional means such as measurement of quartz crystal oscillationfrequency as a function of aluminum deposition the desired thickness ofaluminum is then evaporated atop the lacquer film as is represented byblock 25. In this way the thin aluminum coating which is notself-supporting is maintained as a layer by the thin self-supportinglacquer layer. Subsequently, the entire assembly is then placed in anoven where the assembly is baked in an air atmosphere at a tempera tureof about 325 Centigrade for approximately one to two hours as isrepresented by block 26. It is noted that this and other processing maypermit the aluminum to oxidize'on its surfaces, forming aluminum oxide,an electrical insulator. While unconfirmed, as hereinafter becomesapparent the existence of the oxide does not detract from and possiblyenhances the operation of the invention.

The lacquer vaporizes during baking and disappears while the aluminumlayer sinks down into place atop the microchannel plate. Normalelectrostatic forces assist in retaining the aluminum in place. Thealuminum is in electrical contact with the electrode coating on thefront surface of the microchannel plate.

In operation an image is received at the front faceplate 2 of the lightamplifier 1 illustrated in FIG. 1, and this image is directed upon thesurface of photocathode 5. Photocathode materials produce electroncurrents in proportion to the magnitude of the incident light. Thusphotocathode at its output back surface produces an electron image or,as otherwise stated, an image of electron currents. Electric field E],where E1 Vl/dl, accelerates all the electrons toward the electronmultiplying structure, namely, microchannel plate 7 and aluminum layer15. These electrons are accelerated through the voltage V1. Uponreaching aluminum layer the electrons have been accelerated up to anenergy level of 400 to 1,000 electron volts corresponding to the voltageapplied between photocathode 5 and microchannel plate 7.

As previously discussed the thickness and substance of aluminum layer 15is such as to make the. layer effectively transparent to electrons ofsuch high energy levels in that such electrons either pass through thealuminum layer 15 or knock out a corresponding electron and the electrontravels forward into a correspondingly located one of the tube passages9.

Otherwise stated, the electrons which are generated due to electroncollisions with the front surface of the microchannel plate are lowenergy level electrons typically on the order of 3 to 5 electron volts,a rather small energy level in contrast to the approximately 600electron volt energy of the incident electron traveling from thephotocathode. The low energy electron travels into the aluminum layerand therein loses what little energy is has due to interaction andcollision with the multitude atoms and electrons in the aluminum layerand is therefore unable to pass through the length of aluminum layerinto one of the passage openings. By contrast, any conductive materialplated or otherwise formed on the front surface of the microchannelplate which, for example, is the electrode element previously described,which is found on the plate in the form provided by the manufacturer,does not cover the entire open ends of the passages and does not presentany barrier to scattered electrons. Further, because the plate 7 islocated close to the photocathode, permitting use of voltages on theorder of 600 volts, higher accelerating voltages which create scatteredelectrons of higher energy levels that might pass through layer 15 areavoided.

For clarity of explanation the path of one electron, e1, serves toillustrate the conventional operation of the microchannel plate 7.Electron e1 is derived from pho tocathode 5 due to the incidence oflight, )u', at the indicated location on the photocathode and isaccelerated toward the electron multiplier. The electron goes throughtransparent"'metal'layer l5 and then into a passage 9 where it comesunder the influence of electric field, E2, established by source 19.Because of its random transverse travel the electron collides with thepassage walls. The passage walls are coated with high secondary emissionmaterial having a secondary emission coefficient of at least 2, or, inother words, greater than one at average impact velocity. Thus theelectron knocks out at least two additional electrons and these, inturn, accelerate and travel, due to electric field E2, in general towardthe back of the passage. As illustrated in the example in FIG. 1 thesetwo electrons having random velocity vector angles, in turn, strike thewalls of the passage at a subsequent location and, in turn, knock outfour electrons. By suitable choice of the length and diameter of thetubes this process of increasing the quantity of electrons continues.And a large number of electrons exit from the back side of microchannelplate 7. Hence, the initial electron e1 which entered the front of theplate is amplified or multiplied many, many times. Upon exit from therear of the passage 9 in microchannel plate the electrons enter highlevel electric field, E3. The electric field accelerates the electronsto a large energy level, through 5,000 volts typically, and they traveland pass throughrnetal backing l6 and strike the surface of the phosphorscreen or target 13, as variously termed, at a correspondingpredetermined location. With conventional electron focusing systemsbetween the plate and target this location can be varied, but in theembodiment illustrated, it is a direct corresponding location. As isconventional, the phosphor emits light, )1 0, in proportion to theamount of electron bombardment and thus the initial low level of lightresponsible for the generation of the single electron from photocathode5 is amplified to a much higher light level or brightness at thephosphor screen 13, much higher than the light which would have beenproduced by the single original electron. I

While the foregoing theory of operation has been discussed in connectionwith a single electron, the operation occurs concurrently with all ofthe incident light in the image or mosaic placed over the entire surfaceof the photocathode and with all the passages in the microchannel plateso that the electron image formed at photocathode 5 appears as a lightimage at the display target electrode 13.

Prior to my invention by the addition of aluminum layer 15 to the frontof the microchannel plate 7 -it was possible for an electron such asillustrated by e2 in FIG. 1, to enter a passage in the microchannelplate and knock out a positive ion, Due to the nature of construction ofthe microchannel plate tube this would most likely be a cesium ion or anoxygen ion or a water ion. Electric field E1 which acceleratesnegatively charged electrons toward plate 7, instead acceleratespositively charged ions toward the photocathode. In striking thephotocathode these particular ions would combine with the photocathodematerial to form a different compound, a compound which would notpossess photocathodic properties and lowered or destroyed, eventually,the effectiveness of the photocathode.

Secondly, the mass of an ion is, of course, hundreds of times greaterthan an electron and when accelerated through the electric field possessrelatively large amounts of kinetic energy. Upon striking-thephotocathode the ion releases this energy and erodes the photocathodematerial and reduces its photocathodic properties. I

As a result of the foregoing effects, the light amplifiers lostsensitivity, presented a faded picture with diminishing brightness anddiminishing contrast typically after an operating life of no more than50 to hours.

Instead, in the construction of the invention aluminum layer 15 absorbsor acts as a barrier to and prevents these large mass ions from reachingphotocathode 5, and in this way acts as an ion trap. In addition, anyions or neutral gas molecules originating from any other tube componentsbehind the microchannel plate cannot pass through layer 15.

Image amplifiers under life test have presently been in operation for inexcess of 1,000 hours in contrast to the 50 to 100 hours provided withprior art tubes and this is accomplished without reduction in the gainoverall of the light amplifier. Theoretically, the ultimate increase inthe operating life of the light amplifier expected from this improvedconstruction and which will be demonstrated in the future is expectedtoincrease by a factor of 100 to 1,000 times over that previouslyavailable.

Further consideration of the invention is better illustrated andunderstood in connection with FIG. 2 which shows a cutaway section Afrom FIG. 1. In normal operation of the light amplifier, electrons suchas e3, of 400 electron volts, pass through aluminum layer and enter oneof the tubes 9 in microplate 7.

With the modification to the aluminum layer sug' gested in which a skinof high secondary emission material, suitably aluminum oxide, is appliedto the underside of the aluminum layer, electron e3 as represented inFIG. 2 passes .through the aluminum and strikes the high secondaryemission material. Inasmuch as such material has a secondary emissioncoefficient preferably greater than 2, two electrons, e31 and e32, areshown emerging from the back surface of layer 15 and traveling into thepassage. In this the layer functions in addition as atransmission-emission dynode. However even without the suggestedcoating, it is possible in many instances for such electrons, such ase3, to knock out some secondary electrons.

Assuming momentarily the deletion from FIG. 2 of aluminum layer 15 theprior art microchannel plate light amplifier devices and an attendantdisadvantage of same as well as a prime and unexpected feature of theinvention can be better understood. Electrons such as e4, as representedin FIG. 2, traveling from the photocathode are many times incident uponan edge surface bordering the passages, 9. This is not uncommon. Aspreviously noted, approximately 50 percent of the apparent surface areaof the microchannel plate open to the passage while the other 50 percentrepresents actual material. This represents the manufacturers compromisebetween the desire of as many passages as possible in a given spaceversus the mechanical requirements of rigidity for the multichannelplate element.

In those instances the electron knocks out from the surface one or moreadditional electrons, which are represented by way of example as e41 ande42. These electrons depart the surface and travel at an angle withrespect to the surface of the microchannel plate but with a component ofvelocity in the direction of the photocathode. These electrons are lowenergy level scattered" electrons and, typically, possess energies onthe level of 3 to 5 electron volts. The electric field El previouslydescribed in connection with FIG. 1 decelerates and turns theseelectrons around and they travel into passages in the microchannelplate. In the passages 9 these electrons act and are multiplied insubstantially the same manner as any other electron as previouslydescribed in connection with the electron multiplication properties ofthe microchannel plate.

Unfortunately, because these electrons are scattered" in any directionand of varying low energy levels there is an uncertainty as to which oneof the passages in microchannel plate into which they will travel. In

FIG. 2, electron e41 is shown traveling into one passage while electrone42 is shown traveling into an adjacent passage. As is also apparent,there are additional passages above and below the illustrated passagesin the 3-dimensional body. It is also possible for the energy level ofthe scattered electrons to be such that it can travel, instead, to thenext adjacent tube into which the previously discussed electron e3traveled.

Inasmuch as each of the electrons represents light of a received image,it is apparent that light intended to be positionally located on thephosphor display screen at a position corresponding to the juncture ofthe adjacent passages where e4 is incident the light is, instead,displaced a predetermined position and presented at one location, thatthrough which electron e41 emerges, or, in addition or alternatively, ata second location, the one to which e42 will travel, as well as manyother passages above and below and around those illustrated to which thescattered electrons can travel. Thus it is possible to obtain instead ofa sharp point location on the phosphor screen a rather blurredrepresentation; the contrast or resolution is thus not as great aspossible. This factor is referred to in the design and specification ofcathode ray tubes and light amplifying tubes as a :limiting resolutionfactor and this limiting resolution factor is specified in the number oftelevision lines (scanning lines) per unit of raster height. The limitof resolution is specified as the number of lines on the screen for agiven height which can be viewed before the lines merge and blur andbecome indiscernible. Typically, on prior art microchannel plate lightamplifying tubes the upper threshold of resolution was 400 TV lines perunit of raster height, the raster height being typically four-tenths of1 inch. The capability of resolution of a phosphor screen itself islimited by phosphor spot size and conventionally the display screen'l3of FIG. 1 is inherently capable of a resolution greater than l,000 linesper unit of raster height.

Considering now the incorporation of the aluminum layer 15 in the lightamplifier. As previously described in connection with the operation inFIG. 1 the alumi num screen acted as a barrier to positive ions, whichions were many hundreds of times larger in both mass and volume than anelectron. In addition, quite unexpectedly, the aluminum layer alsoabsorbed or captured the electrons low energy level scattered electronssuch as those generated by secondary emission from the edge surface ofthe microchannel plate. Thus electrons such as e41 and e42 are nowabsorbed or captured within layer 15 covering passage 9 and they cannottravel into the microchannel plate. The accelerating voltage within therange of 400 to 1,000 volts, as previously noted, is low and at mostgenerates a minimum of higher energy secondary scattered electrons thatmight pass through layer 15 to diminish contrast resolution as a higheraccelerating voltage could do. Because the existence of only these lowenergy level scattered secondary electrons was a substantial factorlimiting the resolution of the light amplifier, their eliminationwithout the generation of those of increased energy permits an increasedresolution capability for the light amplifier. In point of fact, in atube constructed in accordance with the teachings of this invention, a30 percent increase was obtained in the limiting-resolution over acorresponding tube constructed without the aluminum layer 15. Ascontrasted with an upper threshold of resolution of 400 TV lines perunit of raster height obtained with prior art tubes the structure of theinvention makes it possible to distinguish 600 lines per unit of rasterheight.

Should any light pass through photocathode and be incident upon themetal layer 15, it will pass through the transparent layer and throughthe microchannel plate without interfering in the operation of the imageintensifier tube. This avoids the problem of reflecting light from layerback to photocathode 5 and generation of electrons in a differentposition possible with an optically opaque thick metal layer and whichcould cause blooming, is avoided.

In the specific example of my invention the layer 15 which forms the ionand electron trap is a metal, aluminum. However other materials of a lowdensity, suitably a specific gravity below 4, can be used as analternative. Boron and beryllium are examples. Although the precedingexamples are metals, I have also found that nonmetals, normallyelectrical insulators, are equally suitable. By way of example, somesuch nonmetals which can be used to form the layer 15 include boroncarbide, aluminum oxide, silicon oxide, boron nitride, silicon dioxide,magnesium oxide and magnesium fluoride. Whatever alternative material isused, the layer 15 is formed in place on top of the microchannel plateby the same processes described previously for the specific example ofaluminum. Thus the foregoing or any equivalent element compound orcomposition of matter may be used which can undergo processing by thepreferred process. Basically, the material should notbe water soluble,should not oxidize at temperatures less than 300 Centrigrade, and do notreduce in a hydrogen atmosphere at temperatures on the order of 435Centigrade.

Moreover, the structure of the invention requires the materials to be ofa low density and hence they must have a specific gravity less than 4.0and, preferably, the specific gravity of the material is in the range of1.0 to 4.0.

As a further consideration and refinement to the invention, it isdesirable that the materials used as the ion and electron trappinglayer, such as layer 15, possess a high secondary emission coefficientsuitably greater than 3.0 at 400 volts. This insures greater output forelectron multiplication processes and renders the resulting tube ofhigher quality and less susceptible to noise. This characteristic isexhibited by most of the specific examples given.

As previously described, the front edge surface of the microchannelplate 7 as obtained from the manufacturer is electrically conductive.Hence if layer 15 is an electrical conductor it will be in directcontact with the front edge of the microchannel plate, and, accordingly,the voltage V1 which is applied to the microchannel plate and thephotocathode 5 creates a voltage drop across a distance d1 slightlyshorter by the distance d3, the thickness of the layer, and,accordingly, a slightly greater voltage gradient El, which is equal toVl/dl. However inasmuch as the thickness of the layer is insignificantin relation to the distance d1 between the photocathode 5 andmicrochannel plate, for all practical purposes the effect of thethickness of the microchannel plate on the established electric fieldsmay be disregarded. Thus, where the covering layer 15 is of a nonmetalwhich is not electrically conductive but electrically dielectric, thevoltage extends across the space between photocathode 5 and front edgesurface of microchannel plate 7. However as in the preceding case, thethickness of the layer is so small relative to the distance between thephotocathode and microchannel plate that its effect upon the voltageapplied therebetween or the voltage gradient E1 thereacross may bedisregarded. Hence, with either type of construction 1 may refer to thepotential difference or voltage between the photocathode and the frontend of the modified microchannel plate as that between the photocathodeand the front edge of the microchannel plate, disregarding the existenceof the thin layer 15, and likewise the electric field gradient E1established between the photocathode and microchannel plate is the samefor all practical purposes as that gradient established by disregardinglayer 15. And thus it is understood where I describe a voltage or fieldbetween the microchannel plate and photocathode that such voltage orgradient may actually appear across an insignificantly foreshortenedspace in the case where layer 15 is an electrically conductive metal andis intended to cover such structure; Likewise, essentially the same istrue of the distance between the rear of the microchannel plate and thetarget electrode. Hence where I refer to a voltage V3 between the twoelements or a field E3, (VS/d2), between those two elements it is'understood such language is intended to cover the distance between themicrochannel plate and the metal backing layer on the target electrode.

The foregoing detailed description and illustration of the preferredembodiment of my inventions are presented solely for purposes ofexplanation and not by way of limitation. As is apparent to thoseskilled in the art many modifications, substitutions and equivalents tothe foregoing details can be made without departing from the spirit andscope of my invention. lt is therefore understood that the inventionsare to be broadly construed limited only by the breadth and scope of theappended claims.

What is claimed is:

1. An image intensifier which includes:

front end means for receiving an optical image;

a photocathode for receiving said optical image and generating electronsrepresentative of said image;

a target electrode, said target electrode including a layer of metalcovering a back side of said target electrode;

an electron multiplyingmicrochannel plate located between said targetelectrode and said photocathode with said microchannel plate having afront side facing said photocathode and a back side facing said backside of said target electrode, said microchannel plate spaced by a firstpredetermined distance from said photocathode and spaced by a secondpredetermined distance from said target electrode, said secondpredetermined distance being at least 1.5 times greater than said firstpredetermined distance;

a very thin substantially optically transparent nonself-supporting layerof material having a thickness in the range of 50 A. to 400 A. situatedupon and covering the front side of said microchannel plate fortrapping-ions and low energy level electrons and passing other electronsand incidental light to thereby increase the contrast resolutioncapability of the tube and the operational life thereof;

means for applying a first predetermined voltage between saidphotocathode and said microchannel plate;

means for applying a second predetermined voltage between saidmicrochannel plate and said target electrode; said second predeterminedvoltage being more than twice as large as said first predeterminedvoltage;

and means for appying a third voltage between the front and rear ends ofsaid microchannel plate.

2. The invention as defined in claim 1 wherein said material comprises aspecific gravity in the range of 1.0 to 4.0.

3. The invention as defined in claim 2 wherein said material comprises ametal.

4. The inventionas defined in claim 3 wherein said metal comprisesaluminum.

5. The invention as defined in claim 2 wherein said material consists ofa member selected from the group comprising aluminum, aluminum oxide,boron, beryllium, boron carbide, silicon oxide, silicon dioxide,magnesium oxide, and magnesium fluoride.

6. The invention as defined in claim 2 wherein said material possesses acoefficient of secondary emission greater than 3.0 at a potential of 400volts.

7. The invention as defined in claim 1 wherein said first predetermineddistance is in the range of 0.005- inches to 0.020-inches and whereinsaid second predetermined distance is in the range of 0.030-inches to0.050-inches, and wherein said first predetermined voltage is in therange of 400 to 1,000 volts and wherein said second predeterminedvoltage is in the range of 3,000 volts to 8,000 volts.

8. A light amplifier which comprises:

a phosphor display screen of a predeterminedarea;

a metal backing layer covering the backside of said phosphor displayscreen;

a photocathode spaced from said display screen for emitting electrons inresponse to incident light;

a microchannel plate type electron multiplying element located betweensaid display screen and said photocathode, said electron multiplyingelement having a front input side facing said photocathode and a rearoutput side facing'said display screen, said electron multiplyingelement being spaced from said photocathode by a first predetermineddistance within the range of 0.005-inches to 0.020- inches and beingspaced from said display screen by a second predetermined distancegreater than said first predetermined distance in the range of0.030-inches to 0.050-inches;

thin non-self-supporting substantially optically transparent layer ofelectrically conductive mate.-

rial coupled to and covering the front end of said microchannel plate;

means for establishing a first voltage difference between saidphotocathode and said front end of said electron multiplying element toaccelerate electrons in a direction from said photocathode toward saidelectron multiplying element;

means for establishing a second voltage difference between the ends ofsaid electron multiplying element to create an electric field foraccelerating electrons in a direction from the front to the back endthereof; and

means for establishing a third voltage difference between the rear endof said electron multiplying element and said display screen foraccelerating electrons toward said target, said third voltage differencebeing at least twice as large as said first voltage difference. 9. Theinvention as defined in claim 8 wherein said first distance comprisesapproximately 0.012-inches and wherein said second distance comprisesapproximately 0.038-inches.

10. The invention as defined in claim 9 wherein said first voltagecomprises approximately 600 volts and wherein said third voltagecomprises approximately 5,000 volts.

l l. The invention as defined in claim 10 wherein said electricallyconductive material comprises aluminum. 12. An image intensifiercomprising:

front end means for receiving an optical image;

a photocathode;

means for directing said optical image upon said photocathode;

a target electrode, said target electrode including a metal backinglayer;

a microchannel plate, said microchannel plate located between and spacedfrom said photocathode and target electrode with the distance betweensaid plate and said photocathode being less than the dis tance betweensaid plate and said target electrode, said microchannel plate having afront end facing said photocathode and a rear end facing said targetelectrode;

first voltage supply means for establishing a low potential differencein the range of 400 to 1,000 volts between said photocathode and saidmicrochannel plate to accelerate electrons toward said microchannelplate; I

second voltage supply means for establishing a high potential differencein the range of 3,000 to 8,000 volts between said microchannel plate andsaid target electrode to accelerate electrons toward said targetelectrode;

third voltage supply means for providing a potential difference betweenthe front and back ends of said microchannel plate to cause electrons totravel from the front end to the back end of said microchannel plate;and

a very thin non-self-supporting layer of electrically conductivematerial situated upon and covering the front side of said microchannelplate for trapping ions and low energy level'electrons;

whereby the contrast resolution capability and the operational life ofthe tube is enhanced.

13. The invention as defined in claim 12 wherein said thinnon-self-supporting layer of electrically conductive material issubstantially optically transparent.

14. The invention as defined in claim 13 wherein said layer comprisesthe material aluminum and is of a thickness within the range of 50 to400 A.

15. The invention as defined in claim 14 wherein said first voltagemeans comprises approximately 600 volts.

16. The invention as defined in claim 15 wherein said second voltagemeans comprises approximately 5,000 volts.

17. In a light amplification device of the type which includes in vacuumin an envelope;

a photocathode for receiving an optical image on a front surface andemitting a corresponding electron image'from its back surface;

material comprises a metal.

a phosphor screen electrode for converting electron images incidentthereupon into a corresponding visual image, said phosphor screenincluding a metal layer covering its rear side;

electron multiplying microchannel plate means cated intermediate saidphotocathode and said phosphor screen electrode, having a front endfacing said photocathode and a rear end facing said phosphor screenelectrode for receiving an electron image at an input end and emitting acorresponding electron image of greater intensity at its I rear end,said microchannel plate being spaced from said photocathode by a firstpredetermined distance in the range of 0.005-inches to 0.020- inches andbeing spaced from said phosphor screen electrode by a secondpredetermined distance, greater than said first predetermined distance,in the range of 0.030-inches to 0.050-inches;

a thin non-self-supporting substantially optically transparent layer ofmaterial situated upon and covering said front face of said microchannelplate for trapping ions and low energy level electrons and passingincident light, said layer having a thickness in the range of 50 to 400A;

first means for providing a first voltage between said photocathode andsaid microchannel plate;

second means for providing a second voltage be-; tween the frontand rearsurfaces of said micro-f channel plate;

third means for providing a third voltage between the rear surface ofsaid microchannel plate and said phosphor screen electrode, said thirdvoltage being at least twice as large as said first voltage.

18. The invention as defined in claim 17 wherein said 19. The inventionas defined in claim 18 wherein said metal comprises aluminum.

20. The invention as defined in claim 19 wherein said thickness of saidlayer is approximately A.

21. The invention as defined in claim 20 wherein said first voltagecomprises approximately 600 volts and wherein said third voltagecomprises approximately 5,000 volts.

' 22. The invention as defined in claim 21 wherein said first distancecomprises approximately 0.0l2-inches and wherein said second distancecomprises approximately 0.038-inches.

23. The invention as defined in claim 17 wherein said first voltage isin the range of 400 to l,000'volts and said second voltage is in therange of 3,000 to 8,000 volts.

24. The invention as defined in claim 23 wherein said material consistsof a member selected from the group consisting of aluminum, boron,beryllium, boron carbide, silicon oxide, silicon dioxide, magnesiumoxide, magnesium fluoride, and aluminum oxide.

25. The invention as defined in claim 17 wherein said material possessesa specific gravity in the range of 1.0 to 4.0.

26. The invention as defined in claim 17 wherein said material possessesa secondary emission characteristic of at least 3 measured at 400 volts.

27. The invention as defined in claim 17 wherein said first voltage andsaid first distance define an electric field, E, in the range of l Xl0to 4 X 10 volts percentimeter and wherein said third voltage and saidsecond distance define an electric field in the range of 3 X 10 to 6 X 10 volts per centimeter.

1. An image intensifier which includes: front end means for receiving anoptical image; a photocathode for receiving said optical image andgenerating electrons representative of said image; a target electrode,said target electrode including a layer of metal covering a back side ofsaid target electrode; an electron multiplying microchannel platelocated between said target electrode and said photocathode with saidmicrochannel plate having a front side facing said photocathode and aback side facing said back side of said target electrode, saidmicrochannel plate spaced by a first predetermined distance from saidphotocathode and spaced by a second predetermined distance from saidtarget electrode, said second predetermined distance being at least 1.5times greater than said first predetermined distance; a very thinsubstantially optically transparent non-selfsupporting layer of materialhaving a thickness in the range of 50 A. to 400 A. situated upon andcovering the front side of said microchannel plate for trapping ions andlow energy level electrons and passing other electrons and incidentallight to thereby increase the contrast resolution capability of the tubeand the operational life thereof; means for applying a firstpredetermined voltage between said photocathode and said microchannelplate; means for applying a second predetermined voltage between saidmicrochannel plate and said target electrode; said second predeterminedvoltage being more than twice as large as said first predeterminedvoltage; and means for appying a third voltage between the front andrear ends of said microchannel plate.
 2. The invention as defined inclaim 1 wherein said material comprises a specific gravity in the rangeof 1.0 to 4.0.
 3. The invention as defined in claim 2 wherein saidmaterial comprises a metal.
 4. The invention as defined in claim 3wherein said metal comprises aluminum.
 5. The invention as defined inclaim 2 wherein said material consists of a member selected from thegroup comprising aluminum, aluminum oxide, boron, beryllium, boroncarbide, silicon oxide, silicon dioxide, magnesium oxide, and magnesiumfluoride.
 6. The invention as defined in claim 2 wherein said materialpossesses a coefficient of secondary emission greater than 3.0 at apotential of 400 volts.
 7. The invention as defined in claim 1 whereinsaid first predetermined distance is in the range of 0.005-inches to0.020-inches and wherein said second predetermined distance is in therange of 0.030-inches to 0.050-inches, and wherein said firstpredetermined voltage is in the range of 400 to 1,000 volts and whereinsaid second predetermined voltage is in the range of 3, 000 volts to8,000 volts.
 8. A light amplifier which comprises: a phosphor displayscreen of a predetermined area; a metal backing layer covering thebackside of said phosphor display screen; a photocathode spaced fromsaid display screen for emitting electrons in response to incidentlight; a microchannel plate type electron multiplying element locatedbetween said display screen and said photocathode, said electronmultiplying element having a front input side facing said photocathodeand a rear output side facing said display screen, said electronmultiplying element being spaced from said photocathode by a firstpredetermined distance within the range of 0.005-inches to 0.020-inchesand being spaced from said display screen by a second predetermineddistance greater than said first predetermined distance in the range of0.030-inches to 0.050-inches; a thin non-self-supporting substantiallyoptically transparent layer of electrically conductive material coupledto and covering the front end of said microchannel plate; means forestablishing a first voltage difference between said photocathode andsaid front end of said electron multiplying element to accelerateelectrons in a direction from said photocathode toward said electronmultiplying element; means for establishing a second voltage differencebetween the ends of said electron multiplying element to create anelectric field for accelerating electrons in a direction from the frontto the back end thereof; and means for establishing a third voltagedifference between the rear end of said electron multiplying element andsaid display screen for accelerating electrons toward said target, saidthird voltage difference being at least twice as large as said firstvoltage difference.
 9. The invention as defined in claim 8 wherein saidfirst distance comprises approximately 0.012-inches and wherein saidsecond distance comprises approximately 0.038-inches.
 10. The inventionas defined in claim 9 wherein said first voltage comprises approximately600 volts and wherein said third voltage comprises approximately 5,000volts.
 11. The invention as defined in claim 10 wherein saidelectrically conductive material comprises aluminum.
 12. An imageintensifier comprising: front end means for receiving an optical image;a photocathode; means for directing said optical image upon saidphotocathode; a target electrode, said target electrode including ametal backing layer; a microchannel plate, said microchannel platelocated between and spaced from said photocathode and target electrodewith the distance between said plate and said photocathode being lessthan the distance between said plate and said target electrode, saidmicrochannel plate having a front end facing said photocathode and arear end facing said target electrode; first voltage supply means forestablishing a low potential difference in the range of 400 to 1,000volts between said photocathode and said microchannel plate toaccelerate electrons toward said microchannel plate; second voltagesupply means for establishing a high potential difference in the rangeof 3,000 to 8,000 volts between said microchannel plate and said targetelectrode to accelerate electrons toward said target electrode; thirdvoltage supply means for proviDing a potential difference between thefront and back ends of said microchannel plate to cause electrons totravel from the front end to the back end of said microchannel plate;and a very thin non-self-supporting layer of electrically conductivematerial situated upon and covering the front side of said microchannelplate for trapping ions and low energy level electrons; whereby thecontrast resolution capability and the operational life of the tube isenhanced.
 13. The invention as defined in claim 12 wherein said thinnon-self-supporting layer of electrically conductive material issubstantially optically transparent.
 14. The invention as defined inclaim 13 wherein said layer comprises the material aluminum and is of athickness within the range of 50 to 400 A.
 15. The invention as definedin claim 14 wherein said first voltage means comprises approximately 600volts.
 16. The invention as defined in claim 15 wherein said secondvoltage means comprises approximately 5,000 volts.
 17. In a lightamplification device of the type which includes in vacuum in anenvelope; a photocathode for receiving an optical image on a frontsurface and emitting a corresponding electron image from its backsurface; a phosphor screen electrode for converting electron imagesincident thereupon into a corresponding visual image, said phosphorscreen including a metal layer covering its rear side; electronmultiplying microchannel plate means located intermediate saidphotocathode and said phosphor screen electrode, having a front endfacing said photocathode and a rear end facing said phosphor screenelectrode for receiving an electron image at an input end and emitting acorresponding electron image of greater intensity at its rear end, saidmicrochannel plate being spaced from said photocathode by a firstpredetermined distance in the range of 0.005-inches to 0.020-inches andbeing spaced from said phosphor screen electrode by a secondpredetermined distance, greater than said first predetermined distance,in the range of 0.030-inches to 0.050-inches; a thin non-self-supportingsubstantially optically transparent layer of material situated upon andcovering said front face of said microchannel plate for trapping ionsand low energy level electrons and passing incident light, said layerhaving a thickness in the range of 50 to 400 A; first means forproviding a first voltage between said photocathode and saidmicrochannel plate; second means for providing a second voltage betweenthe front and rear surfaces of said microchannel plate; third means forproviding a third voltage between the rear surface of said microchannelplate and said phosphor screen electrode, said third voltage being atleast twice as large as said first voltage.
 18. The invention as definedin claim 17 wherein said material comprises a metal.
 19. The inventionas defined in claim 18 wherein said metal comprises aluminum.
 20. Theinvention as defined in claim 19 wherein said thickness of said layer isapproximately 75 A.
 21. The invention as defined in claim 20 whereinsaid first voltage comprises approximately 600 volts and wherein saidthird voltage comprises approximately 5,000 volts.
 22. The invention asdefined in claim 21 wherein said first distance comprises approximately0.012-inches and wherein said second distance comprises approximately0.038-inches.
 23. The invention as defined in claim 17 wherein saidfirst voltage is in the range of 400 to 1,000 volts and said secondvoltage is in the range of 3,000 to 8,000 volts.
 24. The invention asdefined in claim 23 wherein said material consists of a member selectedfrom the group consisting of aluminum, boron, beryllium, boron carbide,silicon oxide, silicon dioxide, magnesium oxide, magnesium fluoride, andaluminum oxide.
 25. The invention as defined in claim 17 wherein saidmAterial possesses a specific gravity in the range of 1.0 to 4.0. 26.The invention as defined in claim 17 wherein said material possesses asecondary emission characteristic of at least 3 measured at 400 volts.27. The invention as defined in claim 17 wherein said first voltage andsaid first distance define an electric field, E, in the range of 1 X 104to 4 X 104 volts per centimeter and wherein said third voltage and saidsecond distance define an electric field in the range of 3 X 104 to 6 X104 volts per centimeter.